JP5393888B2 - Method for detecting normal conducting transition of superconducting wire - Google Patents

Description

The present invention relates to a method for detecting a normal conducting transition of a superconducting wire.
This application claims priority based on Japanese Patent Application No. 2011-057939 filed in Japan on March 16, 2011 and Japanese Patent Application No. 2010-146304 filed on June 28, 2010 in Japan. , The contents of which are incorporated herein.

  Superconducting wire is expected to be applied to superconducting magnets and superconducting cables, such as nuclear magnetic resonance imaging devices, magnetic levitation railways, magnetic bearings, and motors. There is a lot of research to ensure sex.

  Superconductors that make up superconducting wires generally have a critical temperature (the upper limit temperature indicating superconductivity) that is lower than room temperature, so they can be cooled to below the critical temperature using a cooling medium such as liquid helium or liquid nitrogen or a refrigerator. Used. However, even if the superconducting wire is cooled below the critical temperature from the outside, if a normal conduction transition occurs that causes a transition from the superconducting state to the normal conducting state due to thermal disturbance in a part of the superconducting wire during energization, Joule There is a problem (quenching phenomenon) in which heat is generated and the temperature of the superconducting wire rises, the normal conducting transition around it is promoted, and the normal conducting region is expanded.

  In Patent Literature 1, a carbon film is provided on the superconductor to detect a slight temperature rise just before quenching, when the superconductor has transitioned to a normal conducting state due to thermal disturbance or the like, and a minute temperature change is caused from the voltage of the carbon film. A method of detecting is described. However, the method described in Patent Document 1 has a property that the electrical resistance value with respect to the temperature of the carbon film is remarkably large in an extremely low temperature region of about several K (Kelvin) from the temperature of liquid helium (see FIG. 7 of Patent Document 1). It is what you use. For this reason, it is difficult to apply to a high temperature superconductor having a critical temperature of 77K or higher (for example, about 100K).

Patent Document 2 discloses a superconductor quench in which polarized light from a light source is incident on an optical fiber wound around a superconducting wire, a phase difference of polarization from the optical fiber is detected, and an abnormal polarization state of light transmitted through the optical fiber is detected. A detection method is described.
In Patent Document 3 (particularly, the fourth invention thereof), an optical fiber is attached to the outside of the superconducting wire, and the reflected light from the deformed portion of the optical fiber due to mechanical displacement of the abnormal portion in the superconducting wire during energization or the optical fiber A quench detection method for a superconducting wire is described in which transmitted light from the other end is measured to detect abnormality of the superconducting wire.
However, the methods described in Patent Documents 2 and 3 can only determine whether there is an abnormality in the optical fiber by causing the superconducting wire to move due to quenching and increasing the displacement and deformation of the optical fiber. The details of temperature change cannot be measured.

Patent Document 4 and Non-Patent Document 1 describe a temperature measurement method at an extremely low temperature by an optical fiber type temperature sensor using a fiber Bragg grating (FBG). FBG is an optical fiber type device in which a periodic refractive index change (grating) is formed in the core of an optical fiber, and selectively reflects a specific wavelength (Bragg wavelength) determined by the refractive index of the core and the period of the grating. It has the property to do.
In Patent Document 4, a coating material (coating) such as aluminum (Al) or polymethyl methacrylate (PMMA) having a larger thermal expansion coefficient (TEC) than silica, which is the main component of the optical fiber, is provided around the FBG of the optical fiber. The sensitivity of the temperature sensor is improved by increasing the change in Bragg wavelength due to temperature. Non-Patent Document 1 shows a measurement example of strain, temperature, and linear expansion.

Japanese Patent No. 2577682 Japanese Unexamined Patent Publication No. 8-304271 Japanese Unexamined Patent Publication No. 7-170721 US Pat. No. 6,072,922

Wolfgang Ecke et al., “Fiber optical gratifying sensors for structural health monitoring at cryogenic temperatures”, Proc. SPIE, 2007, Vol. 6530, 653002.

In Patent Document 4 and Non-Patent Document 1, an FBG provided with a coating material is used, for example, as shown in FIG. It is only described that the temperature of the medium can be measured when the FBG provided with the coating material is immersed in a homogeneous medium as shown in FIG.
Moreover, when using FBG described in patent document 4 for the temperature measurement of a high-temperature superconductor, since the high-temperature superconductor formed into the wire is poorly deformable, the superconducting wire cannot be disposed around the entire periphery of the coating material. . That is, the medium around the FBG provided with the covering material cannot be made homogeneous. In addition, considering the thermal conductivity from the wire, even if the coating material around the optical fiber is in close contact with the superconducting wire, the coating material has a difference between the thermal expansion coefficient of the coating material and the superconducting wire. Expansion and contraction is restricted. For this reason, there is a possibility of adversely affecting temperature measurement accuracy and response speed.

  The present invention has been made in view of the above circumstances, and detects a temperature change associated with a normal conduction transition with good accuracy and responsiveness, and a superconducting wire in which the normal conduction transition occurs based on the temperature change of the superconducting wire. It is an object of the present invention to provide a method for detecting a normal conduction transition of a superconducting wire capable of more accurately detecting the state of the above.

In order to solve the above problems, the present invention employs the following configuration.
The method for detecting a normal conduction transition of a superconducting wire according to the first aspect of the present invention is a method for detecting a normal conduction transition of a superconducting wire comprising a base material, a superconducting layer having a critical temperature of 77 K or higher, and a metal stabilizing layer. An optical fiber having a plurality of fiber Bragg gratings formed in a core along a longitudinal direction of the core is bonded and fixed to the superconducting wire; a change in Bragg wavelength of the fiber Bragg grating with respect to a temperature change of the superconducting wire. Measure in advance and obtain a relational expression for measuring the temperature of the superconducting wire from the change of the Bragg wavelength; obtain the temperature change of the plurality of fiber Bragg gratings before and after the normal conduction transition occurs in the superconducting wire. The time difference at which the temperature rise of the plurality of fiber Bragg gratings starts and the plurality of fibers Driver based on the spacing of the Bragg grating, and calculates the propagation speed of the normal conductive transition.
In the method for detecting the normal conduction transition of the superconducting wire according to the first aspect of the present invention, Tmax is the maximum temperature measured in any of the plurality of fiber Bragg gratings, and the origin of the normal conduction transition from the fiber Bragg grating. The maximum temperature at the starting point of the normal conduction transition is calculated by (L / V) υ + Tmax, where L is the distance to the part, ν is the temperature rise rate in the fiber Bragg grating, and V is the propagation speed of the normal conduction transition. May be.
In the method for detecting the normal conduction transition of the superconducting wire according to the first aspect of the present invention, the maximum temperature Tmax and the temperature rise rate υ are determined using a fiber Bragg grating closest to the starting point of the normal conduction transition. You may measure.
In the method for detecting a normal conducting transition of the superconducting wire according to the first aspect of the present invention, a temperature rise rate in each of the plurality of fiber Bragg gratings is obtained from a temperature change of each fiber Bragg grating, and the temperature rise rate is obtained. May be confirmed that the normal conduction transition has propagated to the position of the fiber Bragg grating.
In the method for detecting a normal conducting transition of a superconducting wire according to the first aspect of the present invention, the predetermined threshold value may be set in advance for each current value to be passed through the superconducting wire.

The method for detecting a normal conduction transition of a superconducting wire according to the second aspect of the present invention is a method for detecting a normal conduction transition of a superconducting wire comprising a base material, a superconducting layer having a critical temperature of 77K or higher, and a metal stabilizing layer. An optical fiber having a plurality of fiber Bragg gratings formed in a core along a longitudinal direction of the core is bonded and fixed to the superconducting wire; a change in Bragg wavelength of the fiber Bragg grating with respect to a temperature change of the superconducting wire. Measure in advance and obtain a relational expression for measuring the temperature of the superconducting wire from the change of the Bragg wavelength; obtain the temperature change of the plurality of fiber Bragg gratings before and after the normal conduction transition occurs in the superconducting wire. The temperature rise rate in each of the plurality of fiber Bragg gratings is calculated for each fiber. Calculated from the temperature change of the Bragg grating; checked by whether the temperature increase rate is above a predetermined threshold, whether the normal conductive transition is propagated to the position of the fiber Bragg grating.
In the method for detecting a normal conduction transition of a superconducting wire according to the second aspect of the present invention, a fiber farthest from a starting portion of the normal conduction transition among fiber Bragg gratings determined to have propagated the normal conduction transition. The range where the normal conduction transition has occurred may be estimated by doubling the distance to the Bragg grating.
In the method for detecting a normal conducting transition of a superconducting wire according to the second aspect of the present invention, the predetermined threshold value may be set in advance for each current value to be passed through the superconducting wire.
In the method for detecting a normal conducting transition of a superconducting wire according to the second aspect of the present invention, the superconducting wire is coiled, and a change in the Bragg wavelength of the fiber Bragg grating due to electromagnetic force generated by the coil is measured in advance. And applying the Bragg wavelength change obtained by subtracting the Bragg wavelength change of the fiber Bragg grating due to the electromagnetic force to the relational expression, so that the plurality of fiber Bragg gratings before and after the normal conduction transition occurs in the superconducting wire The temperature change may be obtained.
The optical fiber in which the plurality of fiber Bragg gratings are formed includes a broadband light source, a spectroscopic element, measurement light from the broadband light source being incident on the optical fiber, and Bragg reflected light from the plurality of fiber Bragg gratings. May be connected to a temperature measuring device that includes an optical component that enters the spectral element and a light receiving element that receives light dispersed by the spectral element and outputs a voltage signal.

According to the method for detecting a normal conduction transition of a superconducting wire according to an aspect of the present invention, the change in the Bragg wavelength of the FBG with respect to the temperature change of the superconducting wire is measured in advance, and the temperature of the superconducting wire is measured from the change in the Bragg wavelength. By measuring the temperature change of the superconducting wire using the relational expression, it becomes possible to detect the temperature change accompanying the normal conduction transition with high responsiveness.
By forming a plurality of FBGs along the longitudinal direction of the optical fiber and measuring the time difference at which the temperature rise started in these FBGs, the propagation speed of the normal conduction transition is determined from this time difference and the interval between the plurality of FBGs. Can be calculated.
By measuring the maximum temperature measured in any FBG, the distance from the FBG to the starting point of the normal conducting transition, the temperature rise rate in the FBG, and the propagation speed of the normal conducting transition, the starting point of the normal conducting transition It is also possible to determine the maximum temperature at.
When the normal conduction transition propagates to the position of the FBG, it is also possible to determine whether the normal conduction transition has propagated from the temperature rise speed of the FBG by utilizing the fact that the temperature rise speed becomes larger than the case where it does not. Is possible.
Of the FBGs that have been determined to have propagated the normal conduction transition, it is also possible to estimate the range in which the normal conduction transition has occurred based on the distance from the starting point of the normal conduction transition to the farthest FBG.
When the superconducting wire is coiled and electromagnetic force is generated in this coil, the change in Bragg wavelength due to electromagnetic force is converted into temperature change by using the result of subtracting the change in Bragg wavelength of FBG due to electromagnetic force. Therefore, the temperature change occurring in the superconducting wire can be determined more accurately.

It is a conceptual diagram explaining the method of calculating | requiring the maximum temperature in the origin part of the normal conduction transition which concerns on one Embodiment of this invention. It is a conceptual diagram explaining the method to determine whether the normal conduction transition propagated through the position of FBG which concerns on one Embodiment of this invention. It is a block diagram which shows an example of the temperature measuring device using several FBG. It is sectional drawing which shows an example of the state which bonded and fixed the optical fiber on the metal stabilization layer of a superconducting wire. It is a graph which shows an example of the relationship between the applied frequency of the spectroscopic element (AOTF) used in the Example, and the transmitted light wavelength. It is a graph which shows the example of a measurement of the reflection spectrum of a plurality of FBGs used in the example. It is a block diagram which shows an example of the detection apparatus of the normal conduction transition used in the Example. It is a top view which shows an example of the positional relationship of the heater and FBG which generate | occur | produce a normal conduction transition. It is a graph which shows the example of a measurement of the temperature shift of the wavelength shift and sensitivity in FBG1. It is a graph which shows the example of a measurement of the relationship between the wavelength shift from 77K in FBG1, and absolute temperature. It is a graph which shows the example of a measurement of the relationship between the wavelength shift from 77K in FBG2, and absolute temperature. It is a graph which shows the example of a measurement of the relationship between the wavelength shift from 77K in FBG3, and absolute temperature. It is a graph which shows the example of a measurement of the relationship between the wavelength shift from 77K in FBG4, and absolute temperature. 13 is a graph illustrating the relationship between wavelength shift from 77K and absolute temperature shown in FIGS. It is a graph which shows the example of a measurement of the temperature change of each FBG in Example 1 of the detection method of a normal conduction transition. It is a graph which shows the example of a measurement of the temperature change of each FBG in Example 2 of the detection method of a normal conduction transition. It is a graph which shows the example of a measurement of the temperature change of FBG4 in Example 3 of the detection method of a normal conduction transition. It is a graph which shows the relationship between the square value of the electric current which flows into the superconducting wire in Example 3 of the detection method of a normal conduction transition, and a temperature rise rate. It is a graph which shows the relationship between the electric current which flows into the superconducting wire in Example 4 of the detection method of a normal conduction transition, and the wavelength shift of each FBG. It is a graph which shows the wavelength shift of each FBG in Example 4 of the detection method of a normal conduction transition. It is a graph which shows the wavelength shift of each FBG in the section of -2 to 12 second of Example 4 of the detection method of a normal conduction transition. It is the graph which subtracted the wavelength shift by electromagnetic force from the wavelength shift of each FBG in the section of -2 to 12 seconds of Example 4 of the detection method of a normal conduction transition, and calculated | required the wavelength shift by a temperature change. It is a graph which shows the example of a measurement of the temperature change of each FBG in the section of -2 to 12 seconds of Example 4 of the detection method of a normal conduction transition.

Hereinafter, an embodiment of the present invention will be described with reference to the drawings.
FIG. 3 shows an example of the temperature measuring instrument 10 using the optical fiber 3 in which a plurality of fiber Bragg gratings 4 are formed. FIG. 4 shows an example of a state in which the optical fiber 3 is bonded and fixed on the metal stabilizing layer 1 c of the superconducting wire 1.

<Superconducting wire>
The superconducting wire 1 includes at least a substrate 1a, a superconducting layer 1b having a critical temperature of 77K or higher, and a metal stabilizing layer 1c.
The base material 1a applicable to the superconducting wire 1 of this embodiment can be used as a base material for a normal superconducting wire, and may be high strength. Moreover, in order to make it a long cable, it is preferable that it is a tape form, and the base material which consists of a metal provided with the heat resistance required for the film-forming process of a superconductor, etc. is preferable. Examples thereof include various metal materials such as silver, platinum, stainless steel, copper, nickel alloy such as Hastelloy (registered trademark), or a base material in which ceramics are arranged on these various metal materials. Among various heat resistant metals, nickel alloys are preferable. Especially, if it is a commercial item, Hastelloy (trade name made by US Haynes Co., Ltd.) is suitable, and Hastelloy B, C, G, N, W, which have different amounts of components such as molybdenum, chromium, iron, cobalt, etc. Any type can be used. What is necessary is just to adjust the thickness of the base material 1a suitably according to the objective, and it is 10-500 micrometers normally.

The superconductor constituting the superconducting layer 1b may be a known superconductor as long as it has a critical temperature of 77 K or higher. Specifically, REBa ₂ Cu ₃ O _y (RE is Y, La, Nd, Sm, And a superconductor having a composition represented by a rare earth element such as Er or Gd. The as superconducting layer, and the like can be exemplified _{Y123 (YBa 2 Cu 3 O 7} -X) or _{Gd123 (GdBa 2 Cu 3 O 7} -X). Further, other oxide superconductors, for _{example, Bi 2 Sr 2 Ca n-} 1 Cu n O 4 + 2n + in _δ a composition or the like may be used superconductor consisting of high other oxide superconductors having a critical temperature represented.
The thickness of the superconducting layer 1b is not particularly limited, but is preferably about 0.5 to 5 μm and preferably has a uniform thickness.

The superconducting layer 1b is formed by sputtering, vacuum deposition, laser deposition, electron beam deposition, pulse laser deposition (PLD), ion beam assisted deposition (IBAD), chemical vapor deposition (CVD), etc. Among them, the PLD method and the IBAD method are preferable from the viewpoint of productivity.
The thermal coating decomposition method (MOD method), in which a metal organic acid salt is applied and then thermally decomposed, is applied on a substrate with a solution in which an organic compound of a metal component is uniformly dissolved and then thermally decomposed by heating. This is a method for forming a thin film on a substrate, and does not require a vacuum process, and is capable of high-speed film formation at a low cost. Therefore, it is suitable for manufacturing a long tape-shaped superconducting conductor.

  The metal stabilization layer 1c laminated on the superconducting layer 1b is made of a highly conductive metal material. When the superconducting layer 1b attempts to transition from the superconducting state to the normal conducting state, the current of the superconducting layer 1b is changed. Functions as a commutation bypass. The metal material constituting the metal stabilization layer 1c is not particularly limited as long as it is a material having good conductivity, but is relatively inexpensive, such as copper alloys such as copper and brass (Cu-Zn alloy), stainless steel, and the like. It is preferable to use copper, and copper is more preferable because it has high conductivity and is inexpensive. This makes it possible to increase the thickness of the metal stabilizing layer 1c while keeping the material cost low, and the superconducting wire 1 that can withstand accidental current can be obtained at low cost. The thickness of the metal stabilizing layer 1c is preferably 10 to 300 μm. The metal stabilization layer 1c can be formed by a known method, for example, a sputtering method or a method of soldering a metal tape such as copper.

One or two or more arbitrary layers selected from a diffusion prevention layer, a bed layer, an intermediate layer, a cap layer, and the like may be interposed between the substrate 1a and the superconducting layer 1b.
The diffusion prevention layer is formed for the purpose of preventing the diffusion of the constituent elements of the substrate, and is made of silicon nitride (Si ₃ N ₄ ), aluminum oxide (Al ₂ O ₃ ), rare earth metal oxide, or the like. The diffusion prevention layer is formed by a film formation method such as sputtering, and has a thickness of 10 to 400 nm, for example.
The bed layer is formed for the purpose of reducing the interfacial reactivity and obtaining the orientation of the film disposed thereon. For example, yttrium oxide (Y ₂ O ₃ ), silicon nitride (Si ₃ N ₄ ), oxidation composed of aluminum _(Al 2 _{O 3)} or the like. The bed layer is formed by a film forming method such as a sputtering method, and has a thickness of 10 to 200 nm, for example.

The intermediate layer is formed from a biaxially oriented material in order to control the crystal orientation of the superconducting layer. The intermediate layer may have either a single layer structure or a multilayer structure, and preferable materials include, for example, Gd ₂ Zr ₂ O ₇ , MgO, ZrO ₂ —Y ₂ O ₃ (YSZ), SrTiO ₃ , CeO ₂ , Y _2. Examples thereof include metal oxides such as O ₃ , Al ₂ O ₃ , Gd ₂ O ₃ , Zr ₂ O ₃ , Ho ₂ O ₃ , and Nd ₂ O ₃ . Sputtering, vacuum deposition, laser deposition, electron beam deposition, ion beam assisted deposition (IBAD), physical vapor deposition such as chemical vapor deposition (CVD), thermal coating decomposition (MOD), etc. It can laminate | stack by the well-known method of these. The thickness of the intermediate layer can be adjusted as appropriate, but is usually preferably in the range of 0.005 to 2 μm.

The cap layer is formed through the process of epitaxial growth on the surface of the intermediate layer, then grain growth (overgrowth) in the lateral direction (direction parallel to the plane) and selective growth of crystal grains in the in-plane direction. A capped layer is preferred. Since such a cap layer has a higher degree of in-plane orientation than an intermediate layer made of a metal oxide layer, it is preferable to form a superconducting layer on the cap layer. The material of the cap layer is not particularly limited as long as it can exhibit the above functions, but specifically, preferred examples include CeO ₂ , Y ₂ O ₃ , Al ₂ O ₃ , Gd ₂ O ₃ , and Zr ₂ O. ₃ , Ho ₂ O ₃ , Nd ₂ O _{3 and the} like. Further, a Ce-MO oxide in which part of Ce in CeO ₂ is substituted with another metal atom or metal ion may be included.

  Between the superconducting layer 1b and the metal stabilizing layer 1c, as a metal stabilizing base layer, it has good conductivity such as Ag, and the contact resistance (electrical resistance between the interfaces) with the superconducting layer 1b is low, so that it is familiar. A layer made of a metal material can also be formed. The metal stabilizing base layer can be formed by a known method such as sputtering, and the thickness is preferably 1 to 30 μm.

<Optical fiber>
The optical fiber 3 is bonded and fixed on the metal stabilization layer 1c to the superconducting wire 1 of the present embodiment. The optical fiber 3 may be a known optical fiber capable of forming a fiber Bragg grating (FBG), and is preferably a silica-based single mode optical fiber. The materials constituting the silica-based optical fiber are pure silica glass, quartz glass using an additive that increases the refractive index, such as germanium (Ge), and quartz using an additive that decreases the refractive index, such as fluorine (F). It is possible to select appropriately from glass or the like. In addition, a coating material can be provided around the clad so as to be concentric with a cross section. Specific examples of the covering material include high Young's modulus resins such as polyimide, and metals such as copper (Cu) and nickel (Ni), which can be selected in consideration of adhesion to the adhesive layer 5 described later. it can.

  The optical fiber 3 may be bonded and fixed in contact with or in proximity to the metal stabilizing layer 1c of the superconducting wire 1. The material of the adhesive layer 5 is preferably a material that has excellent durability at low temperatures and a high Young's modulus so that the optical fiber 3 can be protected even at low temperatures. Examples of the adhesive layer 5 include resins such as polyimide and bonding metals.

  As a method for forming FBG in the core of the optical fiber 3, when the core is made of silica glass doped with Ge, for example, krypton fluoride (KrF) excimer laser, SHG (second harmonic) of argon (Ar), or the like. And a method of inducing a change in the refractive index at a predetermined interval along the longitudinal direction of the core by a phase mask exposure method or a two-beam interference exposure method.

The period (interval) of the grating (diffraction grating) can be set in consideration of the refractive index of the optical fiber so that Bragg reflection occurs within the wavelength range of the measurement light. For example, when 1.5 μm band measurement light is used in a silica-based optical fiber, the grating period is preferably about 0.5 μm.
The length of one grating (grating length) can be appropriately selected according to the desired reflectance and reflection band, and is preferably 1 to 10 mm, for example.

The condition for causing Bragg reflection is 2Λsinθ = Nλ, where λ is the spacing of the diffraction grating, λ is the wavelength in vacuum of the light, n is the refractive index of the optical fiber core, incident angle θ, and N is any positive integer. Therefore, when θ is a right angle (sin θ = 1) and the integer N is 1, there is a relationship of λ _B = 2nΛ between the lattice spacing Λ and the Bragg wavelength λ _B.

When a temperature change occurs in the FBG, the Bragg wavelength λ _B changes as the refractive index n and the lattice spacing Λ change. The change of the refractive index n depends on the material of the optical fiber core and hardly depends on the covering material of the optical fiber 3 or the object to be bonded and fixed. On the other hand, since the change in the lattice spacing Λ occurs due to deformation (stretching) along the longitudinal direction of the optical fiber 3, it depends on the coating material of the optical fiber 3 and the object to be bonded and fixed.
If the optical fiber 3 in which the FBG is formed is bonded and fixed to the superconducting wire 1, the linear expansion coefficient of the member constituting the superconducting wire 1 is small even if the linear expansion coefficient of the silica glass constituting the optical fiber 3 is small. Due to the large size, large linear expansion occurs when the temperature rises. That is, when the temperature rises, the lattice spacing Λ of the FBG is extended by the linear expansion of the superconducting wire 1, and the Bragg wavelength λ _B is shifted to the long wavelength side.

<Temperature measurement method for superconducting wire>
In the temperature measurement method of the superconducting wire used in the present embodiment, the Bragg wavelength of the FBG with respect to the temperature of the superconducting wire 1 in a state where the optical fiber 3 having a plurality of FBGs formed on the core is bonded and fixed on the metal stabilizing layer 1c. The change is measured in advance, and a relational expression for measuring the temperature of the superconducting wire 1 is obtained from the change of the Bragg wavelength. Thereby, at the time of measurement, the temperature of the superconducting wire 1 can be measured in real time from the change of the Bragg wavelength.

Since this relational expression is a function representing the correlation between the change in Bragg wavelength and the temperature of the superconducting wire 1, a function representing the temperature dependence of the Bragg wavelength change (ie, the correlation between the temperature and the change in Bragg wavelength) is measured in advance. Can be derived as its inverse function.
The temperature range in which the temperature dependence of the Bragg wavelength change needs to be measured in advance is a predetermined temperature range in which the temperature measurement of the superconducting wire 1 is required, and the temperature at which the superconducting wire 1 is operated in the superconducting state. It is preferable to include the range and the temperature range that can be reached after the normal conducting transition of the superconducting wire 1 occurs.

The temperature measuring instrument 10 shown in FIG. 3 is incident on the optical fiber 3 with the optical fiber 3 in which a plurality of fiber Bragg gratings 4 are formed, the broadband light source 11, the spectroscopic element 13, and the measurement light from the broadband light source 11. The optical component 12 which injects into the spectroscopic element 13 the Bragg reflected light from the some fiber Bragg grating 4, and the light receiving element 14 which light-receives the light disperse | distributed by the spectroscopic element 13 and outputs a voltage signal are provided. The incident side near the light source 11 of the optical fiber 3 is connected to the optical component 12 of the temperature measuring instrument 10.
When measuring the temperature dependence of the Bragg wavelength change, the Bragg wavelength is measured while changing the ambient temperature with a refrigerator or the like with the optical fiber 3 adhered and fixed on the metal stabilizing layer 1c of the superconducting wire 1. .

  The light source 11 is preferably a broadband light source that can arbitrarily output the entire Bragg wavelength range that each FBG can take during temperature measurement. When the wavelength range of the measurement light necessary for temperature measurement is wide, it can be dealt with by combining a plurality of light sources having different output wavelength ranges and causing light to enter the optical fiber 3 from an appropriate light source.

  The optical component 12 interposed between the broadband light source 11 and the optical fiber 3 is not limited as long as it has a function of allowing measurement light from the broadband light source 11 to enter the optical fiber 3 and incident Bragg reflected light to the spectroscopic element 13. It is not limited. A specific example is a circulator. Although the optical component 12 may be a coupler, it is preferable to provide an isolator that transmits light only in the direction from the light source 11 to the optical component 12 in order to prevent the reflected light from returning to the light source 11.

As shown in FIG. 3, the measurement light is incident on the optical fiber 3 from the light source 11, and the spectrum of the reflected light is measured. In order to distinguish the Bragg wavelengths of a plurality of FBGs, the Bragg wavelength values may be different from each other. Since the Bragg wavelength has temperature dependence, it is preferable to provide a wavelength difference that does not overlap the range of Bragg wavelengths that each FBG can take during temperature measurement. The spectrum of the reflected light can be measured as the wavelength dependence of the light intensity by receiving the reflected light with the light receiving element 14 via the spectroscopic element 13 whose transmission wavelength can be arbitrarily selected within a predetermined wavelength range. It is.
As a means for replacing the temperature measuring instrument 10 described above, a configuration in which a tunable laser is used as the light source 11 and Bragg reflected light is input to the light receiving element 14 without using the spectroscopic element 13 can be used. In this case, it is desirable that the tunable laser sweeps the wavelength within a range in which the Bragg wavelengths of the plurality of FBGs can be measured.
In addition, by configuring the temperature measuring instrument 10 based on a known time division multiplexing (TDM) method or an optical frequency domain reflection measurement (OFDR) method, FBGs having the same Bragg wavelength are used for temperature measurement of the superconducting wire 1. You can also.

  As described above, the optical fiber 3 is bonded and fixed to the superconducting wire 1, and the Bragg wavelength changes due to the linear expansion of the superconducting wire 1 as the temperature changes. Generally, the temperature dependence of the linear expansion coefficient of the base material of the superconducting wire and the metal material constituting the metal stabilizing layer is approximated by a polynomial (the superconducting layer has a sufficient cross-sectional area compared to the base material and the metal stabilizing layer). Therefore, the coefficient of linear expansion of superconducting wires is governed by the characteristics of these metallic materials). For this reason, when the relational expression indicating the temperature dependence of the Bragg wavelength is expressed by a simple linear expression, it is difficult to sufficiently approximate a wide temperature range from a temperature lower than the critical temperature of the superconductor to about room temperature. Therefore, it is preferable to use a high-order mathematical expression such as a quartic expression because a single temperature can approximate a wide temperature range with high accuracy. Instead of expressing this relational expression by one mathematical expression, it is also possible to divide the definition area into a plurality of small ranges and express it by a polygonal line function using a different primary expression for each small range.

  3 can also be used for continuously measuring the Bragg wavelength of each FBG of the optical fiber 3 during operation of the superconducting wire 1. Since the relational expression representing the correspondence from the Bragg wavelength to the temperature is obtained in advance, the Bragg wavelength measured in real time can be immediately converted as the temperature of the superconducting wire 1.

When the superconducting wire 1 is energized below the critical temperature, since the superconducting layer 1b is in a superconducting state and has a resistance value of 0, current flows through the superconducting layer 1b and the superconducting wire 1 does not generate heat.
In the superconducting wire 1, if for some reason a superconducting transition occurs in which the superconducting layer 1 b transitions from the superconducting state to the normal conducting state, a resistance is generated in the superconducting layer 1 b, and the current is stabilized with a relatively small resistance. Flow through layer 1c. At that time, in the metal stabilizing layer 1c, Joule heat corresponding to the current value and the resistance value is generated, and heat is generated.

  The linear expansion of this superconducting wire occurs with a very fast time constant against heat generation. In the superconducting wire 1 of this embodiment, even if a normal conducting transition occurs, the optical fiber 3 in which a plurality of FBGs are formed is bonded and fixed to the metal stabilizing layer 1c of the superconducting wire 1, A temperature change (temperature increase) inside the superconducting wire 1 can be detected with good accuracy and responsiveness. That is, it can be said that the temperature measurement method of the superconducting wire according to the present embodiment is a very responsive method. Of course, the optical fiber 3 may be bonded and fixed to the surface of the substrate 1a or the side surface of the superconducting wire 1 as long as the linear expansion of the superconducting wire 1 is sufficiently transmitted.

  If the normal conduction transition of the superconducting wire 1 occurs at one location, the Joule heat generated at the starting point portion 2 conducts to the surroundings and further causes the normal conduction transition, so that the range of the normal conduction transition is the length of the superconducting wire 1. Magnify on both sides along the direction. In the present embodiment, since a plurality of FBGs are provided along the longitudinal direction of the optical fiber 3, as shown in FIG. 1 and FIG. 2, temperature changes (time) at a plurality of different positions from the starting point 2 of the normal conduction transition. Change in temperature with the passage of time). Then, by comparing temperature changes in the plurality of FBGs, it becomes possible to analyze in detail the occurrence state and propagation state of the normal conduction transition.

《Propagation speed of normal conduction transition》
As shown in FIG. 1, when the temperature change of each FBG is continuously measured from before to after the normal conducting transition occurs in the superconducting wire 1, the time when the temperature rise of each FBG starts can be obtained. Specifically, if the temperature change in the time range before the normal conduction transition occurs is considered as the baseline, the start of the temperature rise is a line representing the temperature change (temperature rise) after the normal conduction transition occurs (or its (Tangent line) is obtained from the position on the time axis where it intersects the baseline.

  The propagation speed of the normal conduction transition can be calculated from the time difference at which the temperature rise of the plurality of FBGs starts and the interval between the plurality of FBGs. The interval between the plurality of FBGs is known because a preset value can be used when the FBG is formed in the core of the optical fiber. When the number of FBGs is two, the quotient obtained by dividing the FBG interval by the time difference of the temperature rise start corresponds to the propagation speed of the normal conduction transition. When the number of FBGs is three or more, the propagation speed of the normal conduction transition can be similarly determined by a statistical method such as taking an average.

<Estimation of maximum temperature>
Since the superconducting wire is cooled using a cooling medium or a refrigerator, the temperature of the superconducting wire before the occurrence of the normal conducting transition is approximately the same at various places. In addition, since the material and structure of the superconducting wire are substantially uniform over the entire length thereof, the rate of temperature increase at the location where the normal conducting transition has occurred is approximately the same at each location. The temperature rise rate is a temperature difference that rises per unit time, and can be obtained, for example, by averaging time derivatives of temperature changes.
Therefore, the maximum temperature Tmax _FBG in the FBG in which the normal conduction transition has propagated is that the temperature of the superconducting wire 1 before the occurrence of the normal conduction transition is T ₀ , the temperature rise speed due to the normal conduction transition is υ, and the temperature rise at the FBG position. the time difference from the start until temperature reached a maximum as a Delta] t _1, it can be determined by approximation by _{Tmax FBG ≒ T 0 + υΔt 1} .

In addition, the time difference Δt ₀ from when the normal conducting transition first occurs at the starting point 2 of the normal conducting transition to when the temperature starts to rise at the FBG closest to the starting point 2 indicates that the propagation velocity V of the normal conducting transition is constant. Then, by the distance L from the starting portion 2 of the normal conductive transition to the FBG, it can be obtained by Δt ₀ = L / V.
Furthermore, the end of the temperature rise occurs almost simultaneously throughout the entire superconducting wire because it is due to common causes such as the restriction or interruption of the energization of the superconducting wire 1. That is, the time difference is Δt ₀ + Δt ₁ from the first occurrence of the normal conduction transition until the temperature rise is completed.

When the temperature rise process of the superconducting wire in which the normal conduction transition has occurred is modeled in this way, at the starting point 2 of the normal conduction transition, from the first occurrence of the normal conduction transition to the end point of the temperature rise (Δt ₀ + Δt ₁ ), and the temperature rise continues at the temperature rise rate υ. Therefore, the maximum temperature Tmax _{O at} the starting point 2 of the normal conduction transition is Tmax _O ≈T ₀ + υ × (Δt ₀ + Δt ₁ ) = (T ₀ + υΔt ₁ ) + υΔt ₀ ≈Tmax _FBG + υ × (L / V).

That is, the maximum temperature measured in any of the plurality of FBGs is Tmax (corresponding to the above Tmax _FBG ), the distance from this FBG to the starting point 2 of the normal conduction transition is L, the temperature rise rate in this FBG is υ, and the normal conduction The maximum temperature Tmax _{O at} the starting point 2 of the normal conducting transition can be calculated by (L / V) ν + Tmax, where V is the propagation speed of the transition.
In addition, since the value of the temperature rise speed υ in the FBG may be slightly different depending on the distance from the starting part 2, in order to obtain the maximum temperature at the starting part 2 of the normal conduction transition with higher accuracy, It is preferable to use the temperature change measured by the FBG closest to the starting point portion 2.

《Propagation range of normal conduction transition》
As shown in FIG. 2, when the FBG (FBG-C in FIG. 2) does not reach the far-off FBG because the temperature rise is completed in a relatively short time, the FBG (FBG-C in FIG. 2) Heat generation due to heat conduction may be measured. However, since the heat generation due to heat conduction is smaller than the heat generation when the normal conduction transition occurs at the position of the FBG, both can be distinguished by comparing the temperature rise rate.

Therefore, the temperature increase rate in each of the plurality of FBGs is obtained from the temperature change of each FBG, and whether or not the temperature increase rate is equal to or higher than a predetermined threshold value, the normal conduction transition occurs at the position of the FBG. It can be determined whether or not the propagation has occurred.
As described above, when estimating the propagation speed and maximum temperature of the normal conduction transition, it is necessary that the normal conduction transition propagates to the position of the FBG used for estimation. If the temperature rise rate in the FBG used is equal to or higher than a predetermined threshold value, it can be confirmed that the normal conduction transition has propagated to the position of the FBG.

  In addition, among the FBGs that are determined to have propagated the normal conduction transition, the range where the normal conduction transition has occurred can be estimated by doubling the distance from the starting point 2 of the normal conduction transition to the farthest FBG. . In the example shown in FIG. 2, the FBG farthest from the starting point 2 of the normal conducting transition is FBG-B, so the propagation range of the normal conducting transition is about twice the distance from the starting point 2 to the FBG-B. Can be estimated.

  As described above, the heat generation due to the normal conduction transition is mainly Joule heat according to the current value flowing through the metal stabilization layer 1c and the resistance value of the metal stabilization layer 1c, and the amount of heat generation strongly depends on the current value. The predetermined threshold value for determining whether or not the normal conducting transition has propagated to the position of the FBG is preferably set to a different value for each current value to be applied to the superconducting wire.

  In addition, since heat generation due to heat conduction does not end the rise in temperature directly by limiting or cutting off the energization of the superconducting wire 1, the time during which the heat generated by the heat conduction shows the maximum temperature is at any place. It is not always constant. From this point of view, it is possible to distinguish the propagation range of the normal conduction transition from the range in which only the heat conduction occurs.

《Superconducting protection device》
The detection method of the normal conduction transition of the superconducting wire according to the present embodiment can be used for a protection device for a superconducting wire during operation.
The superconducting protection device receives an electrical signal output from the light receiving element 14 in the temperature measuring instrument 10 of FIG. 3, and automatically analyzes it according to the above-described temperature measuring method or normal conducting transition detection method. , A control device that controls the amount of current applied to the superconducting wire by limiting (decreasing) or shutting down (stopping) the current when an abnormality is detected, an alarm device that issues an alarm to the operator when an abnormality is detected, and operation A status display device, a temperature history recording device, and the like can be provided. Thereby, even if a normal conduction transition should occur, the superconducting wire 1 can be prevented from being blown out or burned out, and the superconducting wire 1 can be protected in a good state.

In particular, by measuring the maximum temperature at the starting point of the normal conduction transition and the range where the normal conduction transition occurred, damage to the superconducting wire due to the normal conduction transition can be quantitatively recognized, confirming safety. After that, if the damage is minor, the operation can be resumed without requiring replacement of the superconducting wire.
Further, even when it is necessary to check or replace the superconducting wire, it is possible to obtain the preliminary information on the degree of damage and perform the work, so that a quicker and more accurate work can be performed.

《Superconducting coil》
This embodiment can also be applied to a superconducting coil that can generate an electromagnetic force (hoop stress) by making a superconducting wire into a coil shape and passing a current through the superconducting wire.
The superconducting coil may be, for example, a pancake-type coil body obtained by bending a superconducting wire in its thickness direction and winding it many times concentrically. Further, two or three or more coil bodies may be laminated.
In this case, when converting the Bragg wavelength shift into the temperature of the superconducting wire using the relational expression for measuring the temperature of the superconducting wire from the Bragg wavelength shift (change in Bragg wavelength), input (substitution) into this relational expression. Instead of applying the Bragg wavelength shift including the Bragg wavelength shift of the FBG due to the electromagnetic force to the relational expression, the Bragg wavelength shift resulting from subtracting the Bragg wavelength shift of the FBG due to the electromagnetic force is used as the Bragg wavelength shift. It is preferable to apply to the relational expression. Thereby, the temperature change which arises in a superconducting wire can be calculated | required more correctly, without converting the Bragg wavelength shift by electromagnetic force as a temperature change.
The Bragg wavelength shift of the FBG due to electromagnetic force depends on the current value flowing through the superconducting wire. For this reason, the current value flowing through the superconducting wire and the Bragg wavelength shift due to the electromagnetic force with respect to the current value are measured under conditions where there is no temperature change beforehand, and the relationship between the current value and the Bragg wavelength shift due to the electromagnetic force is obtained. It is preferable to keep it. The Bragg wavelength shift value due to electromagnetic force can be accurately estimated from this relationship and the value of the current actually energized.

  As mentioned above, although this invention has been demonstrated based on suitable embodiment, this invention is not limited to the above-mentioned example, Various modifications are possible in the range which does not deviate from the summary of this invention.

  EXAMPLES Hereinafter, although an Example demonstrates this invention further more concretely, this invention is not limited to a following example.

(Example 1 of temperature measurement method)
In the temperature measuring instrument 10 of the superconducting wire 1 shown in FIG. 3, the optical fiber 3 is bonded and fixed to the superconducting wire 1 as shown in FIG.
In this example, Hastelloy C276 having a width of 5 mm and a thickness of 0.1 mm was used for the base material 1 a of the superconducting wire 1. For the superconducting layer 1b, GdBCO (GdBa ₂ Cu ₃ O _7-x ) having a width of 5 mm and a thickness of 0.001 mm (that is, 1 μm) was used. This superconducting layer has a critical temperature of about 90K and a critical current of about 230A (temperature 77K, value in a magnetic field 0T environment). For the metal stabilizing layer 1c, copper having a width of 5 mm and a thickness of 0.1 mm was used.

The optical fiber 3 in which the fiber Bragg grating 4 (FBG) is formed has an outer diameter of a core made of quartz glass doped with Ge of about 8 μm and an outer diameter of a clad made of pure silica glass of about 125 μm. It is covered with a coating layer made of polyimide and having an outer diameter of 150 μm.
As shown in FIG. 4, the optical fiber 3 on which the FBG was formed was fixed on the metal stabilization layer 1c using a polyimide resin (HD Microsystems PI2525: model number) to be the adhesive layer 5. More specifically, after temporarily fixing the optical fiber 3 so as to be in close contact with the metal stabilizing layer 1c, a polyimide resin to be the adhesive layer 5 is applied so as to cover the periphery of the optical fiber 3, The polyimide resin was cured by heating at 200 ° C. for 1 minute and fixed.

  In the optical fiber 3 to which the superconducting wire 1 is bonded and fixed, four series of FBGs 1 to 4 are formed at intervals of 10 mm in the longitudinal direction. These FBGs were produced by a known exposure method using a KrF excimer laser and a uniform phase mask. In this example, the grating length of FBGs 1 to 4 was 6 mm. The Bragg wavelengths at room temperature (295 K) and unstrained were prepared at a wavelength interval of about 10 nm with target values of about 1540 nm for FBG1, about 1550 nm for FBG2, about 1560 nm for FBG3, and about 1570 nm for FBG4. The measured value of the Bragg wavelength will be described later together with the measuring method.

  Next, the configuration of a measuring instrument for measuring the Bragg wavelengths of the FBGs 1 to 4 shown in FIG. 3 will be described. This measuring instrument includes a broadband light source 11 that outputs measurement light, a circulator 12, wavelength reference FBGs 1 and 2 (not shown), wavelength reference FBGs 1 and 2, and spectral elements 13 that split Bragg reflected light from FBGs 1 to 4. A photodiode (PD) 14 is generally configured. More specifically, an amplified spontaneous emission (ASE) light source is used for the broadband light source 11 and a wavelength variable filter (AOTF) using an acoustooptic effect is used for the spectroscopic element 13.

  As the ASE light source 11, a light source that outputs light in a wavelength range of 1520 to 1610 nm was used, and the total light output was 50 mW (that is, 17 dBm). The measurement light output from the ASE light source 11 passes through the circulator 12 and enters the wavelength reference FBGs 1 and 2 and the FBGs 1 to 4 of the optical fiber 3 bonded and fixed to the superconducting wire 1. Only the light corresponding to the Bragg wavelength of each FBG is reflected from the measurement light incident on the FBG, and is input to the spectroscopic element 13 via the circulator 12.

The AOTF used as the spectroscopic element 13 is composed of a planar optical waveguide made of LiNbO _3, and by applying a sine wave having a frequency of 160 to 180 MHz to a comb-shaped electrode provided on the upper surface of the waveguide, the AOTF is about 1510 to 1680 nm. Light in the wavelength range can be selectively transmitted. Since there is a unique relationship between the frequency of the applied sine wave and the wavelength of transmitted light, the frequency is swept digitally (stepwise) in the above range with respect to time, and input to the PD 14 with respect to time. The wavelength of the emitted light can be changed in the range of 1510 to 1680 nm. In this embodiment, the step frequency of the frequency sweep (frequency interval at the time of frequency sweeping in a stepwise manner) is 1.5 kHz, the step time (holding time per step) is 4 μs, and a high frequency (180 MHz in the range of 160 to 180 MHz). ) To the low frequency (160 MHz) direction. Under this condition, the time required for one frequency sweep is about 53.3 ms from Equation (1).

  Since this frequency sweep can be performed continuously, the Bragg wavelength of the FBG can be measured at a repetition frequency of 1 / 0.0533 s, that is, 18.75 Hz.

The wavelength reference FBGs 1 and 2 were used to obtain the relationship between the frequency of the sine wave applied to the AOTF and the transmitted light wavelength. The wavelength reference FBGs 1 and 2 were installed in an environment where the temperature change was ± 1 ° C. or less and the strain was ± 10 με or less (however, 1 με = 10 ⁻⁴ %).
The Bragg wavelengths of these wavelength reference FBGs are previously measured to be 1532.100 nm and 1584.500 nm, respectively, at room temperature (295 K) and no strain.

When the applied frequency when the reflected light of the wavelength reference FBGs 1 and 2 is transmitted, the applied frequency is 177.95 MHz with respect to the transmitted light wavelength of 1532.100 nm, and the applied frequency 171 with respect to the transmitted light wavelength of 1584.500 nm. 70 MHz. FIG. 5 is a graph showing the AOTF applied frequency on the horizontal axis and the transmitted light wavelength on the vertical axis. In this way, by complementing the relationship between the applied frequency and the transmitted light wavelength with a linear function, it is possible to precisely associate the applied frequency with the transmitted light wavelength on a one-to-one basis.
In AOTF, the relationship between the applied frequency and the transmitted light wavelength changes due to changes and fluctuations in the temperature environment and the like. In actual measurement, this relationship is calculated for each sweep, and based on this, Bragg of FBG1-4 The wavelength was determined.

  FIG. 6 shows the reflection spectra of the reference FBGs 1 and 2 and FBGs 1 to 4 measured at room temperature (295 K) and no strain using the measuring instrument described above. The Bragg wavelengths of FBGs 1 to 4 determined from this reflection spectrum were FBG1 of 1540.367 nm, FBG2 of 1550.634 nm, FBG3 of 1560.660 nm, and FBG4 of 1569.300 nm.

FIG. 7A shows a schematic configuration of the test apparatus 20 used in this embodiment. 7B, in the superconducting wire 1 in which the optical fiber 3 in which a plurality of fiber Bragg gratings 4 are formed is bonded and fixed, a heater 6 for generating a normal conducting transition of the superconducting wire 1 is provided in the vicinity of the optical fiber 3. An example of the configuration is shown.
The superconducting wire 1 was fixed in a coil shape to a cylindrical jig 24 and cooled in a cryogenic vessel 25. The inside of the cryogenic container 25 is hermetically sealed by a container body 25a and a lid 25b, and is thermally insulated by vacuum evacuation by a vacuum pump (not shown), and the cooling performance of a refrigerator (not shown) that cools the inside of the cryogenic container 25. Is increasing.

The superconducting wire 1 was cooled to a predetermined temperature in the cryocontainer 25 using a refrigerator and a semiconductor temperature sensor (Cellnox thermometer: trade name, not shown) provided thereon.
The optical fiber 3 adhered and fixed to the superconducting wire 1 was taken out of the cryogenic vessel 25 through a vacuum feedthrough that can connect the optical fibers, and the terminal was connected to the temperature measuring instrument 10. The heater 6 installed on the superconducting wire 1 is a heater for heating the superconducting wire 1. When the heater 6 is energized, the temperature of the superconducting wire 1 can be raised to a temperature equal to or higher than the critical temperature of the superconducting layer 1b, and a normal conducting transition can be intentionally generated. In addition, the electrodes 23 provided at both ends of the superconducting wire 1 are connected via a vacuum feedthrough capable of connecting the power cables 22 to each other, and the superconducting wire 1 can be energized from the current terminal of the power source 21.

  Next, the superconducting wire 1 is cooled in the low-temperature container 25, the Bragg wavelengths of FBG1 to 4 in the temperature range of 25K to 295K are measured, and the temperature of the superconducting wire 1 and the Bragg from 295K are based on the obtained Bragg wavelength. The relationship with wavelength shift was obtained. As a representative example, FIG. 8 shows the relationship between the absolute temperature of the FBG 1 and the Bragg wavelength shift, and the sensitivity (the Bragg wavelength shift amount per unit temperature change) obtained from this Bragg wavelength shift characteristic. The sensitivity (indicated by a line without the symbol (♦) in FIG. 8) is the result obtained by approximating the relationship between the absolute temperature (x) and the Bragg wavelength shift (y) shown in Equation (2) with a quartic equation as x. Obtained by differentiating. This result can be expressed by equation (3). The units in the formulas (2) and (3) are absolute temperature (x) K, wavelength shift (y) pm, and sensitivity (y ′) pm / K.

  From the monotonously decreasing Bragg wavelength from 295K to 25K, it was confirmed that the absolute temperature could be uniquely determined from the measured Bragg wavelength shift. Further, since the intercept of y ′ represented by the expression (3) is 4.867, the sensitivity at absolute zero (0K) is 4.867 pm / K. A positive sensitivity at absolute zero indicates that the Bragg wavelength monotonically decreases to absolute zero. That is, the temperature measurement method of the present embodiment can measure up to absolute zero.

Next, the relationship between the absolute temperature and the Bragg wavelength shift obtained in the above experiment was plotted with the horizontal axis representing the wavelength shift from 77K and the vertical axis representing the absolute temperature in order to detect the normal conduction transition at 77K, which will be performed later. . The result in FBG1-4 is shown to FIGS. Further, the results of approximation of the quartic function at this time and the correlation function (R ² ) of this approximate expression are shown in expressions (4) to (7). The units in the formulas (4) to (7) are K for the absolute temperature (y) and nm for the wavelength shift (x).

  Next, for FBGs 1 to 4, the set temperature was substituted into the obtained approximate expression to obtain the measured temperature and its error. The results are shown in Table 1. In any FBG, the measurement accuracy was ± 10 K in the temperature range of 25 to 295 K, and when the temperature range was 77 to 295 K, measurement accuracy of about ± 5 K was obtained. From the above, it is considered that the temperature can be measured with an accuracy of about ± 5K in the normal conduction transition detection at 77K to be performed later. The reason why such a high measurement accuracy is obtained is that equations (4) to (7) are approximated by a correlation function with high accuracy. That is, the temperature measurement method of the present embodiment can achieve extremely high measurement accuracy by obtaining the relationship between absolute temperature and Bragg wavelength shift in advance for each FBG and using an approximate expression determined from this relationship.

(Example 2 of temperature measurement method)
In the above described (Embodiment 1 of the temperature measurement method), the temperature is calculated using the approximate expression obtained for each FBG. However, the temperature may be calculated using one approximate expression given in advance. . Table 2 shows the results of calculating the temperature using the above equations (4) to (7) with respect to the wavelength shift of the FBG 1. Since the equation (4) is a relational expression between the absolute temperature and the Bragg wavelength shift obtained in the experiment using the FBG1, the same result as in Table 1 is obtained, but it is obtained in the experiment using the FBGs 2 to 4. Even using the equations (5) to (7), a measurement accuracy of ± 20K was obtained in the temperature range of 25 to 295K.

  FIG. 13 is a diagram in which the results of FIGS. 9 to 12 are plotted in one graph. FBG1 to 4 indicate that the relationship between the Bragg wavelength shift and the temperature change is very close. Further, in the normal conduction transition detection at 77K to be performed thereafter, it is considered that the temperature measurement accuracy is about ± 20K. Although the temperature accuracy is inferior to the case of calculating from the approximate expression obtained by each FBG in the above (Example 1 of temperature measurement method), by using one approximate expression given in advance, for each FBG This method is useful in that it is not necessary to previously calculate the relationship between absolute temperature and Bragg wavelength shift.

(Example 1 of detection method of normal conduction transition)
Next, using the test apparatus shown in FIG. 7A and FIG. 7B, the normal conducting transition is intentionally generated for the superconducting wire 1 of the present embodiment, the normal conducting transition is detected by FBG, and the temperature generated at this time is detected. Measurement was performed.
In FIG. 14, the superconducting wire 1 is cooled and held at 77 K, which is lower than the critical temperature of the superconducting layer, and the superconducting layer 1 is locally heated by energizing the heater 6 for 3 seconds with a current of 160 A being passed through the superconducting layer. It is the result of measuring the wire temperature at the time with each FBG1-4.

  The FBG 4 adjacent to the heater 6 detected heat generation earliest, and the FBG 1 far from the heater 6 detected heat generation latest. In this embodiment, the time when the FBG 4 adjacent to the heater 6 detects heat generation is defined as 0 seconds, and when the heat generation of the FBG 1 far from the heater 6 is detected and time sufficiently passes, it is 2.1 seconds. The electric current supplied to the superconducting wire 1 is interrupted. This result indicates that the normal conduction transition generated in the heater 6 propagates in the longitudinal direction of the superconducting wire 1. Moreover, the detection time difference of each FBG shows the propagation speed of a normal conduction transition, and the inclination of a wire temperature change shows a temperature rise rate.

  The propagation speed obtained from this result was about 25 mm / s, and the temperature rise rate was about 80 K / s. From these speeds, the maximum temperature generated at the starting point of the normal conduction transition, that is, at the center of the heater 6, can be obtained using the equation (8). In the equation (8), 10 mm is the distance between the FBG 4 and the heater 6, and 245K is the maximum temperature measured by the FBG 4 shown in FIG. Since the result of the equation (8) is 277K and the temperature measurement accuracy at 77 to 295K obtained in advance is ± 5K, it can be estimated that the maximum temperature generated in the wire is about 277 ± 5K.

  As described above, according to the present embodiment, the propagation speed of the normal conduction transition and the temperature rise speed of the superconducting wire can be measured. Further, the maximum temperature at the starting point of the normal conduction transition, that is, the maximum temperature generated in the superconducting wire can be measured from these speeds. From the comparison between the maximum temperature and a preset safety reference temperature, it is possible to determine the necessity for inspection and repair of the superconducting wire.

(Example 2 of detection method of normal conduction transition)
Using the configuration of Example 1, the heater 6 was energized for 1.5 seconds.
FIG. 15 is a result of measuring the wire temperature by FBG when the superconducting wire 1 is locally heated by energizing the heater for 1.5 seconds. In the present embodiment, the time when the FBG 4 detects heat generation is defined as 0 seconds, and the current flowing through the superconducting wire 1 is cut off at 1.1 seconds. The rate of temperature increase was about 80 K / s for FBG4, about 65 K / s for FBG3, about 40 K / s for FBG2, and about 15 K / s for FBG1.

  Since FBG4 and FBG3 close to the heater showed a large temperature rise rate as in Example 1, it can be seen that the normal conduction transition propagated to the positions of FBG4 and FBG3. Since FBG2 and FBG1 far from the heater have a lower temperature rise rate, it can be determined that the temperature rise is not due to heat generation due to normal conduction transition but due to heat conduction from the FBG 4 and 3 portions where normal conduction transition has occurred.

  From this result, it can be estimated that the range showing the normal conducting transition in this embodiment is from the center of the heater to the FBG 3 and the distance is about 20 mm. In addition, since it is considered that this normal conduction transition propagates at the same speed in both directions along the longitudinal direction of the wire from the starting point, it can be estimated that the range in which the normal conduction transition is actually performed is about 40 mm.

  As described above, according to the present embodiment, the normal conducting transition range can be detected. Therefore, even if it is determined that inspection or repair of the superconducting wire is necessary, the range can be limited, and the work efficiency can be improved. In addition, when Example 1 and Example 2 are compared, any one of the FBGs (FBG4) detects heat generation, and then interrupts the current that is applied to the superconducting wire 1 at an early stage. It can be seen that the range of the normal conducting transition can be reduced. That is, when a temperature abnormality is detected, a control device that controls the amount of current supplied to the superconducting wire by limiting (decreasing) or interrupting (stopping) the current, and an alarm device that issues an alarm to the worker when an abnormality is detected By providing an operation status display device, a temperature history recording device, etc., it can be used as a superconducting wire protection device.

(Example 3 of detection method of normal conduction transition)
Using the configuration of Example 1, the current passed through the superconducting wire was changed.
FIG. 16 is a graph showing changes over time in the wire temperature measured by the FBG 4 when the current supplied to the superconducting wire is changed to 160A, 190A, and 220A and the heater is supplied for 1.5 seconds. In FIG. 16, the plot of 160A cites the result of Example 2 (FIG. 15). In the present embodiment, the time when the FBG 4 detects heat generation is defined as 0 seconds, and the current flowing to the superconducting wire 1 is cut off when the FBG reaches 150K or more. The rate of temperature increase was about 80 K / s at 160 A, about 140 K / s at 190 A, and about 160 K / s at 220 A.

FIG. 17 is a graph showing the relationship between the square value of the current passed through the superconducting wire 1 and the temperature rise rate. The rate of temperature increase increased almost in direct proportion to the square value of the current passed through the superconducting wire 1. In a superconducting wire that has undergone normal conducting transition, assuming that all of the energized current (current value is I) flows through the metal stabilizing layer (resistance value is R), the generated Joule heat is directly proportional to R × I ² . That is, the reason why the rate of temperature rise is directly proportional to the square value of the current passed through the superconducting wire is that the heat generated in the normal conducting transition portion is caused by Joule heat generated in the metal stabilizing layer.
For this reason, when it is determined whether or not the normal conduction transition has propagated in the above (Example 2 of the detection method of the normal conduction transition), the temperature increase due to the normal conduction transition increases as the value of the current applied to the superconducting wire increases. It is necessary to consider that the speed increases.
In FIG. 16, the maximum temperature measured on the FBG 4 is about 160 to 170 K, but the maximum temperature at the starting point 2 is the same as in the above (Example 1 of the detection method of the normal conduction transition). As a result of the estimation, a result is obtained that the higher the current value, the higher the maximum temperature at the starting point.

  As described above, according to the present embodiment, it is possible to qualitatively know the mechanism of heat generation accompanying the normal conduction transition of the superconducting wire, and it is possible to quantitatively measure the temperature rise rate and the maximum temperature at this time. .

  In Embodiments 1 to 3 described above, the superconducting wire 1 is fixed in a coil shape, but the present invention is not limited to these embodiments. For example, superconducting that transmits a large current by forming a superconducting wire into a cable. It can also be used for cables.

(Example 4 of detection method of normal conduction transition)
Using the configuration of Example 1, the superconducting wire 1 was cooled and held at 50K, and a 3T magnetic field was applied to an electromagnet (not shown) provided on the outer periphery of the superconducting wire 1. The critical current of the superconducting wire 1 at 50K and 3T is about 200A. In the present embodiment, an electromagnetic force (hoop stress) can be generated by energizing the superconducting wire 1.

FIG. 18 is a graph showing changes in the Bragg wavelengths of the FBGs 1 to 4 when the value of the current supplied to the superconducting wire 1 is changed to 40A, 80A, 120A, and 140A. When the hoop stress is generated, a tensile strain is generated in the longitudinal direction of the superconducting wire 1, so that the tensile strain is transmitted to the FBGs 1 to 4. Therefore, the lattice spacing Λ of the FBG is extended, and the Bragg wavelength λ _B is shifted to the long wavelength side. It has been confirmed that the value of the current supplied to the superconducting wire 1 and the change of the Bragg wavelength are almost directly proportional to the value of the current supplied to the superconducting wire 1, although there is a slight difference between FBGs 1 to 4.

  Next, the current to be applied to the superconducting wire 1 was once interrupted, and after a predetermined time had passed, 100 A was supplied to the superconducting wire 1 again to generate a hoop stress. After a certain period of time had elapsed after the generation of the hoop stress, the heater 6 was energized for 5 seconds to locally heat the superconducting wire 1. FIG. 19 shows the result of measuring the change in the Bragg wavelength of each of the FBGs 1 to 4 at this time. FIG. 20 is an enlarged graph of a section of −2 to 12 seconds in FIG. In the present embodiment, the time when the Bragg wavelength of the FBG 4 has greatly changed (detection of heat generation) is defined as 0 seconds, and the current supplied to the superconducting wire 1 is cut off at the time of 8 seconds.

  In FIG. 19, 100 A is energized to the superconducting wire 1 from −45 seconds to generate a hoop stress, whereby the wavelength shift of the vertical axis increases, and the Bragg wavelengths of the FBGs 1 to 4 shift to the long wavelength side. Can be confirmed. Further, the wavelength shift due to the hoop stress is substantially constant before the FBG detects heat generation (-45 seconds to about 0 seconds in FIG. 19), and the wavelength shift with respect to the current 100A in FIG. You can see that it corresponds. Since this hoop stress is not generated when the current is interrupted, the Bragg wavelength shift caused by the temperature change and the hoop stress generated in the superconducting wire 1 is measured only in the section from -45 seconds to 8 seconds. Therefore, if the obtained Bragg wavelength shift is converted into temperature as it is, the Bragg wavelength shift due to hoop stress is also converted as a temperature change. Therefore, it is desirable to subtract this beforehand to obtain the temperature change generated in the superconducting wire 1. In this embodiment, the Bragg wavelength change in the interval from −30 seconds to −10 seconds, in which no heat generation of FBGs 1 to 4 is clearly recognized, is obtained for each FBG, and this value is subtracted only for the interval from −45 seconds to 8 seconds. did. FIG. 21 shows the result of the Bragg wavelength shift obtained by subtracting the Bragg wavelength shift caused by the hoop stress. Thereby, the temperature change which arises in the superconducting wire 1 can be calculated | required correctly, without converting the Bragg wavelength shift by a hoop stress as a temperature change.

Next, the temperature of the superconducting wire 1 is obtained based on the Bragg wavelength shift obtained in FIG. From the relationship between the absolute temperature and the Bragg wavelength shift obtained in advance in Example 1, the relationship between the wavelength shift from 50K and the absolute temperature is obtained, and a result obtained by approximating this to a quartic function and the correlation function (R ² ) of this approximate expression Is shown in equations (9) to (12). The units in the formulas (9) to (12) are K for the absolute temperature (y) and nm for the wavelength shift (x).

  Using the mathematical formulas (9) to (12) obtained above, the Bragg wavelength shift obtained in FIG. 21 was converted into the temperature of the superconducting wire 1. FIG. 22 is a graph showing the results converted to the temperature of the superconducting wire 1. The temperature change generated in the superconducting wire 1 could be accurately obtained without converting the Bragg wavelength shift due to the hoop stress as a temperature. The propagation speed obtained from this result was about 11 mm / s, and the temperature rise rate was about 25 K / s. Substituting these speeds and the maximum temperature 256K measured by the FBG 4 into the above general formula (L / V) ν + Tmax and calculating in the same manner as the formula (8), the superconducting wire 1 was generated as shown in the formula (13). The maximum temperature was estimated at 284K.

  In this embodiment, an electromagnetic force (hoop stress) was generated by the electromagnet provided on the outer periphery of the superconducting wire 1, but a large coil was produced using a long superconducting wire and a current was passed through the hoop. The present invention can also be used when stress is generated.

DESCRIPTION OF SYMBOLS 1 Superconducting wire 1a Base material 1b Superconducting layer 1c Metal stabilization layer 2 Starting point of normal conducting transition 3 Optical fiber 4 Fiber Bragg grating (FBG)
DESCRIPTION OF SYMBOLS 5 Adhesion layer 10 Temperature measuring instrument 11 Broadband light source 12 Optical component 13 Spectroscopic element 14 Light receiving element

Claims (10)

A method for detecting a normal conducting transition of a superconducting wire comprising a base material, a superconducting layer having a critical temperature of 77K or higher, and a metal stabilizing layer,
An optical fiber in which a plurality of fiber Bragg gratings are formed along the longitudinal direction of the core is bonded and fixed to the superconducting wire;
Measuring a change in Bragg wavelength of the fiber Bragg grating with respect to a change in temperature of the superconducting wire in advance, and obtaining a relational expression for measuring the temperature of the superconducting wire from the change in the Bragg wavelength;
Obtaining a temperature change of the plurality of fiber Bragg gratings before and after the normal conducting transition occurs in the superconducting wire by the relational expression;
Calculating a propagation speed of the normal conduction transition based on a time difference at which the temperature rise of the plurality of fiber Bragg gratings starts and an interval between the plurality of fiber Bragg gratings;
A method for detecting a normal conducting transition of a superconducting wire.   The maximum temperature measured in any of the plurality of fiber Bragg gratings is Tmax, the distance from the fiber Bragg grating to the starting point of the normal conduction transition is L, the temperature rise rate in the fiber Bragg grating is υ, and the normal conduction transition is 2. The method for detecting a normal conducting transition of a superconducting wire according to claim 1, wherein the maximum temperature at the starting point of the normal conducting transition is calculated by (L / V) ν + Tmax, where V is the propagation speed of the superconducting wire.   3. The method for detecting a normal conduction transition of a superconducting wire according to claim 2, wherein the maximum temperature Tmax and the temperature rise rate υ are measured using a fiber Bragg grating closest to the starting point of the normal conduction transition. .   The temperature rise rate in each of the plurality of fiber Bragg gratings is obtained from the temperature change of each fiber Bragg grating, and the temperature rise rate is not less than a predetermined threshold value. The method for detecting a normal conducting transition of a superconducting wire according to any one of claims 1 to 3, wherein the transition is confirmed to propagate.   5. The method for detecting a normal conduction transition of a superconducting wire according to claim 4, wherein the predetermined threshold value is set in advance for each current value to be passed through the superconducting wire. A method for detecting a normal conducting transition of a superconducting wire comprising a base material, a superconducting layer having a critical temperature of 77K or higher, and a metal stabilizing layer,
An optical fiber in which a plurality of fiber Bragg gratings are formed along the longitudinal direction of the core is bonded and fixed to the superconducting wire;
Measuring a change in Bragg wavelength of the fiber Bragg grating with respect to a change in temperature of the superconducting wire in advance, and obtaining a relational expression for measuring the temperature of the superconducting wire from the change in the Bragg wavelength;
Obtaining a temperature change of the plurality of fiber Bragg gratings before and after the normal conducting transition occurs in the superconducting wire by the relational expression;
Obtaining a temperature rise rate in each of the plurality of fiber Bragg gratings from a temperature change of each fiber Bragg grating;
Determining whether or not the normal conduction transition has propagated to the position of the fiber Bragg grating according to whether or not the temperature rise rate is equal to or greater than a predetermined threshold;
A method for detecting a normal conducting transition of a superconducting wire.   Of the fiber Bragg gratings determined to have propagated the normal conduction transition, the range from the normal conduction transition to the farthest fiber Bragg grating is doubled to estimate the range where the normal conduction transition has occurred. The method for detecting a normal conducting transition of a superconducting wire according to claim 6.   The method for detecting a normal conduction transition of a superconducting wire according to claim 6 or 7, wherein the predetermined threshold value is set in advance for each current value to be passed through the superconducting wire. The superconducting wire is coiled, and the change in the Bragg wavelength of the fiber Bragg grating due to the electromagnetic force generated by the coil is measured in advance.
Applying the Bragg wavelength change obtained by subtracting the Bragg wavelength change of the fiber Bragg grating due to the electromagnetic force to the relational expression, the temperature of the plurality of fiber Bragg gratings before and after the normal conduction transition occurs in the superconducting wire The method for detecting a normal conducting transition of a superconducting wire according to any one of claims 1 to 8, wherein a change is obtained.   The optical fiber in which the plurality of fiber Bragg gratings are formed has a broadband light source, a spectroscopic element, and measurement light from the broadband light source incident thereon, and Bragg reflected light from the plurality of fiber Bragg gratings is transmitted to the spectroscopic element. 10. The temperature measuring device according to claim 1, wherein the temperature measuring device includes an optical component incident on the light receiving element and a light receiving element that receives light dispersed by the spectroscopic element and outputs a voltage signal. A method for detecting a normal conducting transition of a superconducting wire.

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